This invention relates to photographic supports and elements, and more particularly, to photographic supports and elements comprising a paper base material having thereon at least two polyolefin coatings containing optical brightener.
Photographic supports and elements comprising a paper base material with polyolefin coatings containing white pigment and optical brightener are a valuable class of photographic materials. Such supports are particularly useful in the preparation of photographic elements such as color prints because they exhibit good brightness and excellent dimensional stability and are highly resistant to the action of aqueous acid and alkaline photographic processing solutions. The polyolefin coating on the base provides a very smooth surface, which is desirable when thin layers, such as silver halide emulsion layers, are to be coated on the base. U.S. Pat. No. 3,411,908 describes such a support which has achieved widespread commercial acceptance.
The purpose of the optical brightener is to make the white areas of the support appear even brighter. The optical brightener fluoresces upon irradiation with UV (ultraviolet) light, emitting visible light, usually bluish in hue, thus enhancing the brightness of the support. Optical brighteners for use in photographic print materials must absorb UV light, especially in the region from 360 to 420 nm, and reemit such light to enhance the brightness of the print. The optical brightener must also be stable to the temperatures as high as from 310 C to 330 C, which are used in incorporating the optical brightener into the polyolefin and in extruding the polyolefin onto the paper base material.
Moreover, the optical brightener must be non-migrating so that it remains in the polyolefin coating and does not exude as a surface film on the polyolefin. Such exudation may not only give rise to a nonuniform brightness of the reflection surface of the support, but also readily transfers to any other surface contacted with it. For example, brightener may be transferred nonuniformly to the back side of the adjacent layer of support when wound in roll form, adversely affecting subsequent coating and finishing operations and, in consequence, the quality and performance of the final element.
U.S. Pat. No. 3,501,298 describes a photographic element having a support comprising a paper base having thereon a polyolefin coating, which contains titanium dioxide and bis(alkylbenzoxazolyl)thiophenes. U.S. Pat. No. 3,449,257 relates to compositions comprising hydrophobic polymers and non-migrating optical brighteners and to paper supports coated with such compositions. The non-migrating optical brighteners are 2,5-bis(benzoxazolyl)thiophenes. U.S. Pat. No. 3,260,715 discloses fluorescent bis(benzoxazolyl)stilbenes, such as 4,4xe2x80x2-bis(benzoxazol-2-yl)stilbene, which are useful as fluorescent brightening agents for textile fibers, papers, resins and photographic color print materials. U.S. Pat. Nos. 4,794,071 and 4,859,539 disclose photographic supports comprised of a paper base having thereon a polyolefin coating containing a white pigment and a mixture of fluorescent bis(benzooxazoyl)-stilbenes. However, obtaining the desired mixture of the fluorescent stilbenes is difficult and very costly. U.S. Pat. No. 5,198,330 and U.S. Pat. No. 5,340,854 disclose improved photographic supports, especially useful for color prints, comprised of a paper base material having thereon a polyolefin coating containing a white pigment and an optical brightener, such as a mixture comprising inclusion compounds of equimolar amounts of a fluorescent bis(benzoxazolyl) stilbene and a cyclodextrin.
However, these and other prior art brighteners do not exhibit the combination of absorption/emission characteristics and brightening power, heat stability, and resistance to brightener exudation to the levels desired for photographic supports and elements. Thus, there remains a need for a photographic element comprising an optically brightened support, such support having improved resistance to brightener exudation and wherein the brightener exhibits excellent absorption/emission characteristics, brightening power and heat stability.
The problem to be solved is reducing the exudation of optical brighteners onto the support surface to allow use of more easily manufacturable/less expensive stilbenes.
The present invention relates to an imaging element comprising an imaging layer and a support, wherein said support comprises a base material having thereon at least two polyolefin layers, wherein the uppermost layer of the at least two polyolefin layers comprises a mixture of non-migratory optical brighteners, the mixture of optical brighteners comprising 2-[4-[2-[4-(2-benzoxazolyl)phenyl]ethenyl]phenyl]-5-methylbenzoxazole, hereinafter referred to as Compound A, having the following formula: 
a 2,2xe2x80x2-(1,2-ethenediyldi-4,1-phenylene)bisbenzoxazole, hereinafter referred to as Compound B, having the following formula: 
and a 2-[4-[2-[4-(2-benzoxazolyl)phenyl]ethenyl]phenyl]-5-dimethylbenzoxazole, hereinafter referred to as Compound C, having the following formula: 
wherein the layers below the uppermost layer of the at least two polyolefin layers comprise migratory optical brightener, and wherein the imaging layer is on the same side of the base material as the at least two polyolefin layers.
The present invention includes several advantages, not all of which are incorporated in a single embodiment. One advantage may be that the photographic support of the invention unexpectedly minimizes exudation of the brightener at the polyolefin surface, when a particular mixture of optical brighteners is incorporated into the top or uppermost pigmented polyolefin layer. Another advantage may be the excellent absorption/emission characteristics, brightening power and heat stability that is achievable with this mixture in the uppermost layer of the present invention to meet the critical requirements of the photographic field. Another advantageous feature of the invention may be that such support exhibits excellent brightness at very low brightener concentration. Yet another advantageous feature of this invention may be the stability of the optical brightener mixture at the temperatures, as high as 310-330 C. Other advantages of the invention will become apparent upon reference to the following description of the preferred embodiments.